The present invention relates to Fourier spectropolarimetry, and more particularly to a high-resolution Fourier interferometer-spectrophotopolarimeter.
In the presence of a scattering medium, a beam of radiation is modified in intensity and polarization. It is known that the effect depends upon two causes: the relative importance of scattering and absorption in attenuating the intensity of hte original beam (the single scattering albedo of the carrier medium), and the scattering diagram of particles in the medium appropriate for both intensity and state of polarization (the phase-matrix of scattering of the medium). These two characteristics are wavelength dependent, and hence the spectral variations of the observed radiation intensity and state of polarization, particularly the corresponding line profiles, can yield information about the medium composition and structure.
The information contained in the intensity variations has been systematically exploited by use of photometers for ultra-low (continuum) spectral resolution studies, and of monochromators for studies of isolated spectral lines at high-resolution. Multiplexing spectrometers, namely Fourier interferometers, have given impetus to these studies because they are capable of effecting high-resolution studies over extended wavelength intervals. However, multiplexing spectrometers measure only the intensity of absorption or emission bands spreading across large wavelength intervals. It has been proposed by the inventor and Krishna D. Abhyankar that an interferometer-polarimeter be developed to study moderate to high resolution spectra of polarization state of absorption or emission bands. (See Fymat, A. L. and K. D. Abhyankar, "An Interferometric Approach to the Measurement of Optical Polarization", Applied Optics, 1970, 9:1075-1081, and Fymat, A. L., "Interferometric Spectropolarimetry: Alternate Experimental Methods", Applied Optics, 1972 11: 2255-2264.) Thus, it has been proposed that studies go beyond obtaining merely the Stokes parameter of light which defines intensity (I) and to obtain the state of polarization (degree of polarization, orientation of plane of polarization, ellipticity of polarization ellipse) across spectrum with any desired resolution achievable by a two-beam, amplitude division interferometer such as Michelson's interferometer.
An instrument based on the work of the present inventor and Krishna D. Abhyankar, as reported in the first publication listed hereinabove, has been disclosed in U.S. Pat. No. 3,700,334. The method on which that instrument is based will hereafter be referred to as a "first method". A simplified version of that instrument, based on the further work of the inventor, was reported in the second reference listed hereinabove and actually built for telescopic observations of solar light diffusely reflected from planetary atmospheres. The method on which the latter instrument is based (referred to hereinafter as the "second method") uses only a single polarizer-analyzer whose transmission axis can have several orientations, as compared to the set of three polarizers in the first method, two of which are of variable orientation as shown in the aforesaid patent. The second method is thus able to analyze much weaker light sources. The instrument has operated at the Steward Observatory, Tucson, Ariz., and the National Mexican Observatory, Baja California, Mexico, to (i) provide the first polarization spectra of the planet Venus in the wavelength of 0.8 micrometers (.mu.m) to 2.7 .mu.m at the full instrument resolution of 0.5 cm.sup..sup.-1 ; (ii) to verify experimentally the existence of the phenomenon of spectral polarization theoretically predicted by the present inventor (see Fymat, A. L., "Polarization in Astronomical Spectra: Theoretical Evidence" in Planets, Stars and Nebulae Studied with Photopolarimetry, editor: T. Gehrels, University of Arizona Press, 1974. 617-636); and (iii) to similarly verify the inventor's theoretical prediction of the effect of polarization on the spectral line shape and on the structure of vibration-rotation bands (see Fymat, op. cit. supra). Complete instrument description and polarization spectra can be found in: Forbes, F. F. and Fymat, A. L., "Astronomical Fourier Spectropolarimetry", in Planets, Stars and Nebulae Studied with Photopolarimetry, op. cit. supra at 637-660. That instrument was for producing interferograms of atmospheric radiation incident on the apparatus. From the interferograms, the four Stokes parameters defining both intensity and state of polarization can be concurrently determined. The parameters are the intensity, I, and O, U and V which gave, respectively: degree of polarization, orientation of the plane of polarization, and ellipticity of the polarization ellipse.
Conventionally, the Stokes parameters can be obtained by making four suitably chosen measurements. For example, total intensity (I) may be obtained by using successively 0.degree. and 90.degree. linear polarizer-analyzers in the usual photopolarimetric arrangement, and summing the corresponding data. Alternatively, it may also be obtained by Fourier transform spectroscopy using an ordinary unmodified Michelson interferometer. The main difference between these two measurements is the considerably higher spectral resolution that can be achieved by the interferometer; it is typically three orders of magnitude finer. Measuring degree of polarization (Q) also requires the use of 0.degree. and 90.degree. polarizer-analyzers identically as above, but now followed by differencing of the corresponding data. The remaining two parameters (U and V) can be obtained by using a 45.degree. linear polarizer-analyzer and a compensator, such as a wave plate which, in the plane transverse to the direction of light propagation, retards the phase of one component of the light electric vector with respect to the other component by a fixed amount, 0.degree. and 90.degree. respectively.
Typically, three of the required four measurements are made by employing a fixed 0.degree.-retarder and a rotating linear polarizer-analyzer whose transmission axis azimuth is successively oriented along the directions 0.degree., 45.degree. and 90.degree., in optical series relationship. The fourth measurement is obtained by using a fixed 90.degree.-retarder and the 45.degree.-polarizer. This is an irreducible set of measurements suggested by the inventor and Krishna D. Abhyankar in "An Interferometric Approach to the Measurement of Optical Polarization", Applied Optics, 1970, 9:1075-1081, who showed that the additional two measurements suggested earlier are redundant. These two measurements would employ a 135.degree.-polarizer and 0.degree.- and 90.degree.-retarder, respectively. See, for example, Born, M., and Wolf, E., Principles of Optics, 4th edition, Pergamon Press, 1970, at page 546. However, except for ordinary Fourier transform spectrometry which considers only the total intensity (I), these conventional techniques do not provide the degree of resolution necessary for analysis of the spectra of, for example, relatively unknown gaseous, liquid, or solid media, or media containing elements in some or all of these phases. On the other hand, if the conventional photopolarimeters are coupled with high-resolution spectrometers (e.g., a Fabry-Perot interferometer) which will act as filters, and the spectral lines in the wavelength range of interest are scanned one by one, then, high-resolution may, in principle, be achieved. However, in these arrangements, the amount of light energy falling on the detector would be so small that the signal-to-noise ratio would be insignificant. Adequate signal-to-noise ratio values and spectral resolutions are both necessary. These two requirements can be achieved only by interferometer-polarimeters.
The instrument described in the aforesaid patent generally includes any standard or conventional two-beam, amplitude division interferometer which is modified by the inclusion of a polarizer in each of the beams and an analyzer positioned in front of a sensor or recording device. More specifically, the system employs a beam-splitter which serves to divide light from a selected light source into a pair of individual light beams. Each of the light beams is directed through a polarizer. The polarizers are positioned to have preselected planes of polarization with respect to each other and with respect to the plane of polarization of the analyzer. The polarized light beams are applied to a variable optical retarder which serves to selectively modify the relative optical path lengths of the light beams. An optical mixer may be employed to recombine the two light beams. The recombined light beams are projected through an analyzer, such as a linear polarizer, to a sensor or recording device. In the case of a Michelson interferometer, the beam-splitter and the mixer form a single instrumental component, and the variable retarder is the interferometer itself. In the inventor's further work, three additional spectropolarimetric methods are discussed. (See Fymat, A. L., "Interferometric Spectropolarimetry: Alternate Experimental Methods", Applied Optics, 1972, 11:2255-2264.) one of these has been discussed hereinbefore as the second method. Another method, which may be referred to as the third method, uses a single linear polarizer-analyzer placed at the entrance of the instrument in the incoming light path, and rotated along the three directions 0.degree., 45.degree. and 90.degree.. In a fourth method, a hybrid of the second and third methods, a linear polarizer is placed in the incoming light path and an analyzer is placed in front of the detector. Here three alternatives are offered where both polarizer and analyzer at at 0.degree., and at 90.degree. successively, and: (i) the polarizer is at 45.degree. and the analyzer at 0.degree.; (ii) the polarizer is at 45.degree. and the analyzer is at 90.degree., or (iii) both the analyzer and polarizer are at 45.degree.. Instead of having both the polarizer and the analyzer successively at 0.degree. and 90.degree., it is equivalent to use an analyzer at 45.degree. and a polarizer successively at 0.degree. and 90.degree.. Unfortunately, the third and fourth methods provide only I, Q, U and not V.
An instrument based on any of the four methods above described is adapted for measuring only the effect on polarization of optical radiation by a scattering medium to gain information about the medium structure and composition, or measuring the polarization of absorption or emission lines in planets, stars and nebulae, in the air-glow spectrum, and in chemical analyses. The incident radiation has already been subjected to scattering by the medium and there is no opportunity to study separately the effect on polarization of both transmission and reflection of light by the particles composing the medium, especially in relation to a reference beam which would have been unaffected by the medium. It would be desirable to provide for laboratory use an instrument adapted to hold a sample cell in a chamber and obtain data on the distribution of energy and complete polarization state across the spectrum of (i) the reference light entering the chamber, by-passing the sample cell and reaching the detecting device; (ii) the same light after a fixed-angle reflection from a medium to be analyzed in the sample cell; and (iii) the same light after direct transmission through the medium to be analyzed, with the spectral resolution and light gathering power of Fourier transform spectroscopy.